The investigation of signed languages is crucial to our understanding of the neurobiology of human language, as well as to our understanding of deafness and the design of policies and programs for deaf individuals. This project is divided into three major sections, each focusing on a different facet of our general research question: What is the impact of the visual-gestural nature of sign language on the characteristics of linguistic systems and their neural and cognitive underpinnings? In Section One (Effects of modality on language processing), we address theoretical claims about the representation of the non-dominant hand in sign language production by examining the nature of compensatory articulation using novel sign block techniques (paralleling bite block experiments for speech). We also investigate the effects of language modality on the expression of spatial information, focusing on how distinct linguistic resources for encoding visual shape and viewpoint affect communication in spoken vs. signed languages. In Section Two (The interplay between language and visual-spatial cognition), our goals are to identify the ramifications of language modality for sign-based memory (Is visual-spatial working memory distinct from linguistic working memory for sign language?); for interactions between language and spatial cognition (Does the spatialized expression of locative relationships affect non-linguistic spatial categorization?); and for how perceptual categories are linguistically coded (Do iconic properties of signed languages affect linguistic encoding of sense information?). In Section Three (The functional neural organization for language), we use transcranial magnetic stimulation and functional Magnetic Resonance Imaging to investigate a) the neural systems underlying sign language production and b) the neural basis for the structural similarity between vision and sign language. We address the following specific questions: What are the optimal parameters for fMRI studies that examine overt sign production? What are the roles of the ventral intraparietal area and the posterior superior parietal lobule in sign production? How do the iconic properties of sign language (specifically, the structural mappings between classifier constructions and elements within a visual scene) impact the neural bases of language comprehension? Our overall aim is to understand the nature of sign language processing, while at the same time addressing issues of theoretical importance within linguistics, psycholinguistics, and cognitive neuroscience that also result in practical applications for deaf individuals.